1. Do you normally use a protective filter over your lenses? If so, is it antireflection coated?

2. Your series of samples is backlighted, with the sky and possibly the sun shining brightly onto the front element of the lens. When you notice this blue spot, do you have a relatively strong source of light outside the image area?

Doesn't seem to me that lens flare would worsen as one stopped down. As someone else has suggested, it would be interesting to see if this phenomenon occurs regardless of light direction. What happens when you shoot a blank white wall? Also would be interesting to see what happens if you used these lenses on a film camera. It surely is bizzare.

On my Canon 350D, the imager assembly doesn't reflect a lot of light but it does have a mirrorlike finish. I am wondering if one of the surfaces in these particular lenses is acting like a concave mirror and is focusing an image of the aperture opening back onto the imager.

This would explain a lot of things, such as:
1. The central spot gets smaller as the aperture opening gets smaller.

2. The apparent brightness of this reflection increases as the diaphram is stopped down. This requires some further explanation. The intensity of this image would not change as the diaphragm is stopped down, though its area would, and the intensity of the expected image would of course decrease. The exposure time would of course increase with the decreasing intensity of the expected image, resulting in a proportionate increase in the contribution of the diaphragm image to the whole as the aperture is reduced.

3. The blue-violet color of the resultant spot because the intensity of the offending lens surface would be fairly strongly colored, especially if the peak spectral intensity is at the extreme red or violet end of the spectrum.

4. The fact that this spot is not noticeable in the viewfinder.

5. The fact that this phenomenon was never noticed with film because film produces a diffuse rather than a specular reflection.

6. The fact that the phenomenon is lens-dependent.

7. Central hot spots simillar to this are sometimes reported when shooting infrared or ultraviolet with digital. This would be expected because the antireflection coatings are less effective outside the visible band and such reflections would be more likely to show up.

If I am correct, then:

1. The central spot's diameter will be approximately proportional to the aperture's diameter.

2. If the shutter speed, ISO rating, lighting, and subject are held constant, then the intensity of the spot will also be (approximately) constant regardless of the aperture. This would be easier to measure if you shot an object like a checkerboard with intensely black areas interspersed with your image. Something like this chart I originally intended for measuring barrel distortion and lateral chromatic aberration:

For this purpose, the regularity of the black squares isn't especially important but their blackness is. You might try pasting bits of black photo paper or velvet onto a white background. This will make measuring the intensity of the blue spot a lot easier. You may get a higher reading at larger diaphragm openings because of ghosting and more conventional lens flare.

3. You should get a similar result when mounting this lens onto another camera model, though the overall brightness of the reflection may be different. If so, you should be able to see the difference when you look at the imagers of both cameras set for a long time exposure or an imager clean.

4. This particular lens would probably be a lousy performer if you shot digital ultraviolet images.

I included a picture taken indoors. The light is coming from a window at the side. The camera is fairly far away from the window and there is a corner in the wall, so there's practically minimal direct light falling onto the camera. Still the centre discoloration is visible, although less obvious.

All shots with this lens were done without a lens hood or skylight (uv) filter. I will do a test with a Hoya HMC Sky filter on the weekend, to see whether there is any effect.

I've checked up a bit on the history of my lens, which is apparently a type II. Which makes it quite old and not all lens elements do have amber coating. So it's not a multicoated lens throughout. I've used it before on my Nikon F2 and it never showed this kind of discoloration, although it's a bit prone to lens flare.

It is correct to say that the discoloration does not appear visibly in the view finder, when stopping down. I would have noticed that, as I need to stop down before shooting. After all your valuable comments, I presume it's the aperture blades interacting with the lens design and the reflective surface of the imager.

So, if you're trying old glass on your new digital toy, it might be worth testing it systematically over the whole aperture range, to avoid surprises!

Let me follow up with you on the skylight filter next week.
Cheers and thanks to all.
Sean

Also, if I am correct, neither the Hoya filter nor a lens hood will have much effect on the central purple spot but may affect general lens flare. The diaphragm itself doesn't have a direct effect other than to provide an object to image and to affect the incoming light. Here is what I think is happening:

When I went digital, I bought the D70s because it would take all my lenses I used on my FM2. As of yet I haven't seen anything like this. I mostly use use my ol' 70/300 zoom and my 50mm prime. I rarely even use the kit lense that came with the camera.

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